First get lanyards, second get pins. Where you get them depends on many things. You can find loads of pins on eBay however many of the pins sold on eBay are scrappers. There is a thread a little ways down that will help you find sellers that many of us have used and been happy with. You can also purchase good pins from mousepinsonline.com, the grab bags are well priced.
Third go to the parks and start trading, trade with CM's and if you feel comfortable trade with individuals. Be warned that you will find some who will tell you that every pin you have is garbage or that will tell you that if you want to trade with them you have to go buy them a brand new LE and give them the receipt. My advice when you encounter these traders is to simply say Thank You and walk away. That's what I do. just walk away, but also know that we are not all like that and many of do want to trade with you and your kids so please do not judge everyone by the actions of the few.
After you get a few pins and a starter lanyard... Have fun. It only gets crazy when it becomes a full hobby and then it gets expensive and you need to watch out. But for day trading have fun and stay with eh CM trades and the books in the shops and resorts
Don't be afraid to approach individuals with pins, we love trading with CM's and people, but do be aware that there are many "sharks" or recently termed "pin nazi's" out there who may try to tell you your pins are all garbage, scrappers or they may tell you that you have to buy them a brand new pin and give them the pin & receipt in trade. It is my advice to simply tell them "Thank You" and to move on. You do not need to purchase a pin for them, you can walk away. If anything feels "off" go with your gut and just walk away.
Oh and be aware that pin trading can be very addicting! lol